This invention relates generally to combustion of gaseous fuels wherein the NOX content in products of combustion or flue gases are reduced to acceptable levels. More particularly, this invention relates to low NOX combustion systems for gaseous fuel fired compact boilers and similarly fired fluid heating devices.
In the above-mentioned copending application, a system of controlling flue gas NOX content through controlling the ratios of injected flue gas, and ambient air, into the primary combustion air. In that application the flue gas is scavenged or intercepted in the boiler exhaust through the use of a novel bell mouthed duct. Final control of the NOX boiler outlet gas emissions is achieved through sensing low NOX level downstream of the flue gas tap.
Although the above-mentioned system is creditable, applicants in continuing investigation have discovered additional methods for reducing NOX, particularly in the "compact" boiler designs. The invention disclosed herein provides a method for reducing NOX in boiler stack emissions that is less complex, easier to adjust and is lower in cost than earlier systems.
Therefore, it is an object of this invention to provide a method and apparatus for reducing the NOX level in compact boiler stack emissions.
It is an additional object of this invention to provide a method and apparatus for reducing compact boiler NOX levels in stack emissions through controlling flue gas injection into the primary and secondary air inputs to the boiler or heater.
It is another object of this invention to reduce the NOX content of compact boiler emissions through control of mixed tertiary air and flue gas injection into the boiler combustion chamber.